Generally, the programmable logic controller needs the expansion module for expansion when the number of the control points of the CPU (Central Processing Unit) module is insufficient or the programmable logic controller requires additional function. The expansion module is in communication with the CPU module. Based on the connection way between the expansion module and the CPU module, the programmable logic controller can be classified into the programmable logic controller with serial connection structure or the programmable logic controller with parallel connection structure.
As regards the programmable logic controller with serial connection structure, the first expansion module is connected with the CPU module in series, and the follow-up expansion module is connected with the preceding expansion module sequentially. As for the programmable logic controller with parallel connection structure, the expansion modules and the CPU module are connected in parallel. Even if any of the expansion modules is removed, the communication and the power transmission among the remaining expansion modules and the CPU module are not affected, and the situation of signal interruption could not happen. There are two approaches of the programmable logic controller with parallel connection structure. Firstly, the programmable logic controller includes a base module having a plurality of base connectors, and connectors are mounted on the CPU module and the expansion modules respectively. By the connection between the base connector and the corresponding connector, the CPU module and the expansion modules are assembled with the base module respectively. Secondly, the programmable logic controller includes a rail-type base with copper rail, and plural connectors are mounted on the CPU module and the expansion modules respectively. By the connection between the copper rail and the connector, the CPU module and the expansion modules are assembled with the rail-type base respectively.
However, the programmable logic controller with serial connection structure still has some drawbacks. In case that any of the expansion modules is broken or is removed as required, the expansion modules serially connected therewith are interrupted from receiving the signal and power. Under this circumstance, the CPU module can't communicate with and control the interrupted expansion modules, and a part of the programmable logic controller stops working. Thus, for example, the temperature control may be lost and the sensors may be disabled, which causes the increasing loss, the decreasing activation and even safety anxiety.
In addition, the programmable logic controller with parallel connection structure also has some drawbacks. For the first approach, the base module can't be expanded. The number of expansion modules is limited by the number of base connectors, which reduces the applicability. Moreover, the base module includes a bracket, a circuit board and a plurality of base connectors, and the construction cost is high. For the second approach, the rail-type base can't be expanded or extended. The number of expansion modules is limited by the length of copper rail, which decreases the applicability. Moreover, the rail-type base is a complex structure base with a bracket and plural copper bars, and the construction cost is high. Furthermore, the exposed copper bar may be short-circuited by the broken copper wire or the tool, and thus breaks the modules.
Therefore, there is a need of providing an expansion module of a programmable logic controller in order to overcome the above drawbacks.